2.) Is it have a sense to join any class action lawsuit? What are the benefits?
One of the main purposes of bankruptcy protection is protecting the near-bankrupt company from lawsuits until it can get back in business and pay its debts; this is the main reason Mt.Gox requested it. If it gets granted, lawsuits will be effectively useless until it remains in place; lawsuits are also blocked until the court decides whether to grant it or not.
Nothing at all can be done until the court decides if bankruptcy protection will be granted or rejected (which should, hopefully, happen in 1-2 months).